May 2011

Abstracts of the QSIT Lunch Seminar, May 5, 2011

Symmetry breaking and fluctuations at the Dicke quantum phase transition

Rafael Mottl, Quantum Optics Group, ETH Zurich

The quantized motion of ultracold atoms coupled to the light field of an optical cavity realizes an effective version of the Dicke Hamiltonian. This system exhibits a zero-temperature phase transition from a normal phase to a superradiant phase which can be controlled by the coupling strength between the atoms and the light field. As the cavity field actively takes part in the coupled system dynamics, the photons leaking out of the cavity provide in-situ information about the system. This allows us to study the symmetry breaking process at the phase transition and to characterize the fluctuations below the phase transition. We observe temporal correlations of the fluctuations with a characteristic and diverging time scale.

Quantum storage of photonic entanglement in a crystal

Félix Bussières, Group of Applied Physics - Optique, Université de Genève

Large experimental efforts are devoted to harnessing entanglement between light and material systems due to its prospective role in future quantum information networks. We demonstrate entanglement between a photon at telecommunication wavelength and a single collective atomic excitation stored in a crystal. One photon from an energy-time entangled pair is mapped onto a crystal and then released into a well-defined spatial mode after a predetermined storage time. The other photon is at telecommunication wavelength and is sent directly through a 50 meter fiber link to an analyzer. Successful storage of entanglement in the crystal is proven by a violation of a Bell inequality. These results are a step towards multiplexed quantum repeaters for long-distance quantum networks.

Reference:

C. Clausen, I. Usmani, F. Bussières, N. Sangouard, M. Afzelius, H. de Riedmatten, N. Gisin, Nature 469, 508-511 (2011)